Why does my 24-105 have a different magnification (macro ratio) from a 105mm macro lens at same MFD?

Started 7 months ago | Questions thread
A Marcus Regular Member • Posts: 301
Re: Why does my 24-105 have a different magnifications

We measure subject distance from a point called the front nodal. Should the lens be a single element symmetrical, this point would fall in the middle of the glass lens. We measure focal length from a point called the rear nodal. Again, if we are dealing with a simple lens this point would be at the same location. Now camera lenses are quite complex. The focal length is measured from the rear nodal to the focused image when imaging a subject at infinity (like a star).

Now a complex lens is constructed using many glass lens elements. Some are cemented to their neighbor, some air spaced. Some are convex and some are concave. The net power of the combination places these two cardinal points in bizarre locations, they are often inverted to make the lens barrel shorter and or to accommodate the mirror swing and or focusing mechanism. To this end, these front nodal might fall in air, forward of the lens barrel. The rear nodal might be shifted rearward to allow for mirror swing and the like.

To achieve unity (magnification 1) the image plane falls two focal lengths behind the rear nodal and the subject distance falls two focal lengths forward of the front nodal. The focal length at 1:1 is the distance subject to image divided by 4.

In other words, these measurements are somewhat complex.

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